Polysilicon gates are widely used in the manufacture of metal-oxide-semiconductor (MOS) devices. In typical polysilicon (polysilicon) gate formation processes, after the formation of a gate dielectric layer and a polysilicon layer, the gate dielectric layer and the polysilicon layer are patterned to form a gate stack, which includes a gate electrode on a gate dielectric.
The patterning of the gate stack may cause damage to the gate electrode and gate dielectric, and thus, adversely affect the integrity of gate dielectric. One of the consequences is that in regions of the gate dielectric close to bottom corners of the gate electrode, a high electrical field may exist, and the adversely affected gate dielectric will cause reliability problems. In addition, the leakage current between the gate electrode and underlying substrate may increase.
To solve the above-discussed problems, a polysilicon re-oxidation process was developed. In a typical polysilicon re-oxidation process, after the patterning of the gate stack, an oxidation process is performed. Accordingly, an oxide layer is formed on the exposed sidewalls of polysilicon gate electrode and silicon substrate. With the polysilicon re-oxidation process, the integrity of gate dielectric is improved, and damage to polysilicon gate is repaired.
The conventional polysilicon re-oxidation process, however, has the adverse effects of prolonging the channel length and thickening gate dielectric, thus the performance of MOS devices is degraded. One example of such degradation is the reduction in drive currents. To compensate for the degradation in performance, a nitrided polysilicon re-oxidation process is performed, in which a silicon oxynitride layer instead of an oxide layer is formed. However, only NMOS devices benefit from the nitrided polysilicon re-oxidation, while the performance of PMOS devices is degraded.
Accordingly, what is needed in the art is a MOS device that may incorporate a silicon oxynitride layer to take advantage of the benefits associated with improved reliability and performance while at the same time overcoming the deficiencies of the prior art.